Conventional switched reluctance machines provide a generally robust structure and low manufacturing cost. However, the majority of the electromagnetic forces used by a conventional switched reluctance machine do not contribute to useful work. Rather, these forces create undesirable vibrations that are a major drawback. Thus, conventional switched reluctance machines have limited industrial applicability. Surface mount permanent magnet synchronous machines offer higher torque density than conventional switched reluctance machines. However, rare earth permanent magnet material is expensive and ineffective placement of the rare earth permanent magnets results in high cost and wasteful use of the permanent magnet material.
For example, U.S. Pat. No. 5,304,882 to Lipo et al discloses a variable reluctance motor with permanent magnet excitation having a single set of stators and a single rotor having permanent magnets. However, the motor in Lipo requires a significant amount of permanent magnet material, thereby making the manufacture of such a motor expensive. Further, the motor is limited by the amount of electromagnetic forces which contribute to rotational motion, thereby limiting the torque density of the motor and its overall efficiency.
Therefore, there is a need for intelligent hybridization of rare earth permanent magnets and effective placement to increase power density at a reduced cost to manufacture. Further, there is a need in the art for a double stator permanent magnet machine in which a higher proportion of the electromagnetic forces generated contributes to motion with a reduced amount of permanent magnet material, thereby lowering the overall cost of manufacture.